The present invention relates to an equalizer for a reproduced waveform of a digital magnetic recording, and in particular, relates to an equalizer for a reproduced waveform reproduced by the use of a thin film head, wherein the equalizer is suitable to eliminate undershot portions of the reproduced waveform, and further suitable to reduce the width of an isolated pulse waveform to thereby improve the resolution equivalently.
In digital magnetic storage devices, much efforts have been made year by year to increase capacity and areal recording density, and in order to achieve the higher areal recording density, approaches have been taken for a higher bit density and a higher track density. Among these, in particular, to promote the higher bit density, thin film heads have been put in practical use.
The thin film head 1 is schematically illustrated in FIG. 1. Generally, in the thin film head, as distinguished from conventional bulk heads, a pole face 47 (a face in an opposed relationship to a recording medium 4) has a limited width with respect to a magnetic transition interval within the recording medium, and thus as shown in FIG. 2, peculiar negative peaks appear in an isolated reproduced waveform.
FIG. 2 shows an isolated pulse waveform 6 when information is recorded on and reproduced from a recording medium with a thin film magnetic head having a thick pole face 47. This waveform 6, as contrasted with an isolated pulse waveform 49 which is a reproduced waveform of information recorded with a thin film head having a thin pole face, has undershot portions at positions 7, spaced from a main peak 48. As a result, an output signal frequency response characteristic 8 of the thick pole face head, as compared with an output signal frequency response characteristic 50 of the thin pole face head, has a wave-like change from a lower recording frequency as shown in FIG. 3 (refer to A. Kakei, M. Oshiki, T. Aikawa, M. Sasaki, T. Kozai, IEEE Trans. Magn., MAG-18, 1131 (1982)). Further, as shown in FIG. 4, a pattern peak shift 9 of the thick pole face head is large as compared with a patter peak shift 51 of the thin pole face head. Further, the pattern peak shift characteristic 9 in case of the thick pole face head has a wave-like change as the frequency increases in contrast to that in case of the thin pole face head.
Therefore, from the view point of the reproducing characteristics such as the frequency response and the pattern peak shift, the thinner pole face length is desirable.
On the contrary, from the view point of the recording characteristics, a small sectional area of a thin film head 1 at a stepped portion 5 causes a limiting factor of the recording magnetic field.
Therefore, a sufficient write field 3 can not be obtained due to the fact that magnetic flux passing through the pole piece layer is limited by magnetic saturation at this stepped portion 5. For this reason, in order to produce a sufficient write field, it is required to increase the sectional area of the pole piece layer at stepped portion 5 of the thin magnetic head and this will result in an increase in the thickness of the pole face 47.
As above mentioned, it is necessary to select the pole face length satisfying the frequency response and the recording field strength simultaneously in designing thin film heads and it becomes more severe in higher bit density.